Compensating filters

Abstract

PCT No. PCT/GB91/02200 Sec. 371 Date Aug. 3, 1993 Sec. 102(e) Date Aug. 3, 1993 PCT Filed Dec. 11, 1991 PCT Pub. No. WO92/10876 PCT Pub. Date Jun. 25, 1992.A prefilter (5) for an audio system comprising a loudspeaker (1) in a room (2), which corrects both amplitude and phase errors due to the loudspeaker (1) by a linear phase correction filter response and corrects the amplitude response of the room (2) whilst introducing the minimum possible amount of extra phase distortion by employing a minimum phase correction filter stage. A test signal generator (8) generates a signal comprising a periodic frequency sweep with a greater phase repetition period than the frequency repetition period. A microphone (7) positioned at various points in the room (2) measures the audio signal processed by the room (2) and loudspeaker (1), and a coefficient calculator (6) (e.g. a digital signal processor device) derives the signal response of the room and thereby a requisite minimum phase correction to be cascaded with the linear phase correction already calculated for the loudspeaker (1). Filter (5) may comprise the same digital signal processor as the coefficient calculator (6). Applications in high fidelity audio reproduction, and in car stereo reproduction.

We claim: 
     
       1. A method of conditioning programmable filter apparatus (5) to filter a signal supplied to an acoustic transducer (1) located in an acoustic environment (2), comprising supplying, for storage in the apparatus, filter parameters derived by a method which comprises the steps of: (a) deriving first data relating to a transducer compensation signal response (F L   -1 ) which, in combination with the signal response (F L ) of the transducer (1), substantially reduces the deviation of the transducer signal response (F L ) from uniformity;   (b) deriving second data relating to an acoustic environment compensation signal response (F R   -1 ) which, in combination with the signal response of the acoustic environment (2) over a path therethrough to a predetermined position therein, substantially reduces the deviation of the acoustic environment signal response (F R ) from uniformity; and   (c) deriving subsequently said filter parameters from said first and said second data.   
     
     
       2. A method according to claim 1, in which the transducer compensation response (F L   -1 ) is derived so that its effect, in combination with the transducer response (F L ) is such as to provide a substantially constant signal group delay. 
     
     
       3. A method according to claim 1, in which the transducer compensation response (F L   -1 ) is derived so that its effect, when in combination with the transducer response (F L ), is such as to compensate phase distortions in the transducer response which are substantially independent of position or direction relative to the transducer (1), whilst leaving substantially uncompensated those phase distortions which are substantially dependent thereon. 
     
     
       4. A method according to claim 1, in which the acoustic environment compensation response (F R   -1 ) is derived by the steps of: generating an acoustic signal within said environment (2) via a transducer (1) for which the transducer compensation response (F L   -1 ) is derived to compensate;   measuring the said signal at a place (3) in the environment (2); and   processing the measured signal to derive the response (F R ) of the acoustic environment.   
     
     
       5. A method according to claim 4, in which the signal processing is responsive to, and is to take account of, data relating to the transducer response (F L ). 
     
     
       6. A method according to claim 1 including the step of generating spectral data relating to the signal response (F R ) of the acoustic environment, and processing said data to generate second data such that said acoustic environment compensation signal response includes substantially less long lasting resonance than the inverse of the acoustic environment signal response. 
     
     
       7. A method according to claim 6, in which the effect of said processing is greater at frequencies above a predetermined threshold (f LOW ). 
     
     
       8. A method according to claim 6 in which the effect of said processing is greater at frequencies below a predetermined threshold (f HIGH ) which is within a frequency range of said filter. 
     
     
       9. A method according to claim 6 in which said processing comprises a step of adjusting the amplitude of spectral coefficients of a magnitude which would give rise to a long lasting resonance. 
     
     
       10. A method according to claim 6 in which processing comprises a step of smoothing said data by processing each spectral datum in accordance with spectrally neighbouring data. 
     
     
       11. A filter conditioned according to claim 1.